It has been recognised for some time that patient controlled drug administration is desirable in many situations of chronic or temporary (such as post operative) pain. Before the advent of patient controlled analgesia, analgesia relied on periodic injections of drugs by a physician or nurse, typically at four hour intervals. This has the disadvantage that for long periods the patient's analgesic level is significantly above or below the optimum level.
Patient controlled analgesia improves on that prior art by enabling the infusion of small quantities of analgesics at regular intervals as perceived to be required by the patient. That prior art has also further developed to include apparatus and devices which allow for the continuous infusion of analgesics, with or without the need for patient control.
One form of such a device is described in the applicant's published international patent application WO 93/00944. In that publication, there is described an apparatus for patient controlled infusion of a drug, the apparatus allowing for intermittent administration of an analgesic or the like, the intermittency of the administration being controlled by the use of a small bore tube in the controlled refilling of a metering device, such that the patient may not administer further doses too rapidly after previous doses.
However, this system does not provide for the apparatus to be also used to continuously infuse analgesics or the like. The apparatus also would not prevent a patient from administering a dose earlier than recommended, albeit a dose of smaller volume than a full dose.
Another problem with prior art patient controllable drug delivery systems relates to the actuation of the metering device that delivers the drug. In general, metering devices in prior art systems must be actuated for prolonged periods to deliver the drug to the patient via relatively narrow conduits or tubing. In this respect, the prior art metering devices must be actuated for a sufficient time to deliver the drug to the patient against the pressure produced by the narrow conduits or tubing. The prolonged actuating periods of the prior art metering devices prevents their use by certain patients who are weakened or otherwise unable to actuate the metering device for a sufficient length of time Furthermore, when the prior art systems are controlled by nursing staff or the like they are required to remain with the patient for as long as it takes to actuate the metering device.
The system of the present invention aims to provide for separately controlled intermittent surges of a drug, such as an analgesic. Preferred forms of the present invention aim to provide for the continuous infusion of the drug as well as for separately controlled intermittent surges of the same drug. The system also aims to provide for the control of the intermittent surges so as to ensure the necessary time delay between surges, particularly where the system is patient controlled, as opposed to being controlled by a physician or by nursing staff. Finally, the present invention also aims to provide a separately controllable (by patient, physician or nursing staff) system with a relatively short actuation time.